A number of natural and artificial gas mixtures contain two or more gases having different weights or densities. For example, such separation processes are useful in large-scale industrial processes such as petroleum refining, air separation, and recovery of carbon dioxide from power plant flue gases for geologic storage, as well as for smaller-scale processes such as isotope separation and chemical analysis. A sufficiently efficient and cost-effective mechanism for such gas separation could also be used for extremely large-scale projects, such as the removal of greenhouse gases, primarily carbon dioxide, from air.
Thermal diffusion occurs in non-isothermal gas mixtures. More specifically, lighter components are preferentially concentrated in the hotter regions, and heavier components are preferentially concentrated in the colder regions. That is, a quiescent gas mixture with a steady temperature gradient also has a concentration gradient. This separation is known as the Soret effect. In other words, if a two-component gas is enclosed in a chamber that is heated on one side and cooled on the other, a compositional gradient will develop in the chamber; with the lighter gas species becoming slightly more concentrated on the hotter side of the chamber, and the heavier species becoming slightly more concentrated on the cooler side. In a simple, static system, however, incomplete separation occurs resulting in only a minor variation in the composition of the gas between the hotter and colder regions. Thermal separation of gases, therefore, is not a practical solution for efficient separation of gas components in a gas mixture.
It is known that combining thermal-diffusive separation with convective flow can result in larger degrees of gas separation than that achievable solely with thermal separation. The best-known mechanism for illustrating this effect is the Clusius-Dickel column. Separation in a Clusius-Dickel column occurs by thermal diffusion that is enhanced by convection. The Clusius-Dickel column is a counter-flow device, meaning that the convecting air streams are moving in opposite directions (in a closed-loop fashion), while also diffusively equilibrating. A Clusius-Dickel column consists of a gas-filled tube cooled on the outside with a water jacket and with a heated wire in the center. However, this radial geometry is most easily conceptualized as a slot, with a hot side and a cool side, as shown in FIGS. 1A and 1B. FIG. 1A illustrates a conventional Clusius-Dickel column with an initial 50-50 mixture of a component A gas which is lighter than a component B gas. A convective flow occurs due to the temperature gradient and the difference in densities in components A and B. FIG. 1B illustrates that following some period of convective flow, the lighter component A concentrates at the top of the column and the heavier component B concentrates at the bottom of the column. As will be understood, the component gases A and B do not sharply separate at the top and bottom sections of the Clusius-Dickel column, but rather a gradation with increasing concentration of component A develops from the center of the column to the top of the column. Likewise a gradation with increasing concentration of component B develops from the center of the column to the bottom of the column.
FIG. 2 schematically illustrates how buoyant convection enhances thermal separation. FIG. 2 schematically illustrates a Clusius-Dickel column bounded by a cold wall on the left side and a hot wall on the right side, wherein the column is theoretically partitioned into square cells, with the gap spanned by two cells. Each of the cells of the Clusius-Dickel column is filled with a binary gas mixture containing 50% of a first lower density (i.e., lighter) gas and 50% of a second higher density (i.e., heavier) gas. In FIG. 2, the numbers indicate the concentration of a hypothetical heavier component in a two-component gas mixture. Thermal diffusion, solely, produces a lateral concentration difference of about 4%, for example. Buoyant convection causes the gas to be transported upward on the right side, leftward at the top, downward on the left side, and rightward on the bottom of the column. If alternating steps of thermal diffusion and buoyant convection in a theoretical model are applied as shown in FIG. 2, and the cell values averaged following these two processes, the average value of concentration of the heavier gas is shown in FIG. 2. After each successive pair of thermal-diffusion and buoyant-convection steps, the vertical concentration variation increases, with the ultimate limit shown in the lower right of FIG. 2. In this highly simplified example, thermal diffusion alone can produce only a 4% difference (i.e., 52% vs. 48% of the heavier gas component), whereas thermal diffusion and buoyant convection in combination can produce a 28% difference (i.e., 64% vs. 36%) from bottom to top, which is a 7-fold improvement. In this example, the separation that ultimately can be achieved is determined by the values assumed for lateral thermal separation (i.e., 52% vs. 48%) and the number of cells in the vertical direction (i.e., 8). In principle, an infinite number of cells in the vertical direction would lead to complete separation at the bottom and top of the column (i.e., 100% vs. 0%, rather than 64% vs. 36%), even when the mass ratio is near unity, as for isotopes. However, infinite time is required to achieve such separation.
The Clusius-Dickel column is a batch system—that is, the column is filled with a fixed volume of gas and allowed to reach a steady state condition over a long period of time. The bulk concentration of such a system does not change; only a tiny amount of the highly enriched gas species of interest can be extracted from the end of the tube. Therefore, it would be desirable to have a cost-effective and fast process for separating a gas mixture containing a lighter gas and a heavier gas, resulting in complete or near complete separation of the two gas components, in a continuously flowing stream of the gas mixture. This would allow continuous sampling of the purified components.